1)Incorrect material selection – The coefficient of thermal expansion for all steel, hardened steels, copper & aluminum, must be taken into consideration when designing and building tools. Large differences can cause damage and uneven wear to all components.
2) Incorrect back plates – Most molds have 2 back plates, one on the fixed side and one on the moving side. Back plates have 3 main functions:
To hold the mold in the molding machine using clamps
To form part of the runner system
To support the entire mold against excessive platen deflection
If the back plates are too thin, the resultant repetitive deflection during each cycle eventually causes the following part quality problems:
Flashing /Short shots / Weight variation / Voids / Sink marks
Balancing issues in the runner system
These quality problems occur because the plates do not provide enough support to stabilize the mold against cavity injection pressure and clamp tonnage.
3) Incorrect runner design – A runner system that is not balanced will cause filling issues and inefficiency in running the tool. Some parts may be over-packed and other under-filled; both scenarios could cause quality issues with the part.
4) Incorrect gate design – Selecting the incorrect gate has a similar effect with filling issues and inefficiency in running the tool. The gate is the area where the polymer transits from the runner to the part, and if this is wrong the gate could cause restrictions, incorrect polymer flow, turbulence, or other defective filling issues.